Brzn-  So you have zero voltage drop anywhere in the system and taillight dims when turn signal lights up?  Perhaps I didn't explain the process clearly but I'm going to say that is not possible. 

I'd say most of the time the voltage drop is right at the battery... a voltmeter across the battery terminals will often show a significant voltage drop when the signals flash. 

It could be the harness is the place where you're getting high resistance...  but you could still measure the voltage drop....  there are several ways to do it.

Brand new battery, NOS flashers, NOS Flasher relay, DEET Pigtail from rear Taillight assembly, NOS Voltage Regulator, new Fuse. all grounds (including at the main coil) cleaned at the frame and what contacts the frame ground points. All wires connected to the Taillight and rear flashers checked out okay with my volt meter. I'm to the point of believing the issue is inside the wiring somewhere; either in the main Harness, the Ignition switch, front or rear brake switch or Flasher, Hi/Low Beam switch. I also have a DEET Front Brake switch to install when I do the main Harness.

Exactly what do you mean by "All wires connected to the Taillight and rear flashers checked out okay with my volt meter"? 

Replacing components is not the "troubleshooting" process I've described. Cleaning grounds is not a bad idea either... not the same as checking for a bad ground though. 

And yes, the problem is likely "inside" one or multiple components (battery, wires, connectors, switches, etc.)... It's not a complete mystery though.  You can measure the voltage drop at multiple points along the circuit with the circuits under load and find the issue. 

I like to think of voltage in wires as pressure inside pipes.  If there is a restriction in a pipe, there will be a pressure drop.  This situation of a taillight dimming is similar to the shower head pressure dropping when someone flushes a toilet. 

If you put pressure gauges in the pipes at various points all over your house, with all the faucets turned off the pressure would be the same everywhere.  Now add load by turning on faucets, and you'll start seeing pressure drops...  if a pipe is partially plugged to a certain faucet, the pressure before the restriction will be fine...  the pressure after the restriction might be fine if the faucet is just cracked open...  but open the faucet all the way and the pressure in the pipe after the restriction will drop dramatically. 

You can demonstrate this to yourself with a garden hose with an adjustable nozzle or even by holding your finger over the end of the hose.  With the nozzle off, barely open the faucet.  If you measured the pressure with no flow, it would be the same before and after the faucet (which in this example is the restriction in the pipe circuit).  Now start letting water flow...  initially the pressure will be high and water will spray everywhere. 

As you open the nozzle on the end of the hose, the pressure will drop after the faucet due to the restriction at the faucet.  Pressure measured before the restriction (barely open faucet in this case) will still be high.

Similarly, if you measure voltage (pressure) at the taillight with no load (flow), the voltage (pressure) will be the same as at the battery.  Now if you turn the turn signals on...  realizing that when the signals light up it's a lot of load (flow) on the wiring (piping) system...  if there is a restriction in the system the voltage (pressure) will drop in the wires after that restriction as the bulb load (flow) increase...  just like the pressure dropped after the barely open faucet that was creating a restriction.  

It's very common in my experience that the battery is actually the restriction on these bikes...  battery voltage (pressure) drops significantly when the signals light up...  easy test.  Just connect a voltmeter across the battery and see what happens to the voltage when the signal bulbs flash on.  It's going to drop some...  but if it drops significantly that's likely the issue.  Anyway, by checking voltages along the wiring and seeing what happens to the voltage at different points when the signals light, you can find the "restriction". 

For example, battery has 6.4 volts when resting (no load).  When turn signals light up, voltage at battery drops to 6 volts. At red wire going into ignition switch, you still see 6 volts when signals light...  but at brown wire coming out of ignition switch the voltage drops to 4 volts when signals flash on and back up to 6.3 volts when signal lights are off.  That means the contacts in the switch are a problem, they are creating a restriction when the flow increases. 

I could write a book on all the possibilities... hopefully this has been enough to illustrate the principle that you can measure the voltage along a circuit while it's under load and pinpoint the restriction(s) that are creating voltage drop(s) in the wiring system. 

Don't gloss over the battery... new or not some just can't do the job.  That's why running with the engine revved sometimes cures the problem.

You can also measure voltage drops directly (digital meters are good for this).  For example, place one voltmeter probe on the red wire into the key switch and the other on the brown wire after the key switch....  meter will tell you the voltage drop across those two points...  again, it will be zero with no load...  but under load the restriction will show up.

 

I'll mention one more thing that can be deceiving... a brighter bulb will always be brighter as voltage drops.

Taillight may be on at full brightness when signals light up... then taillight seems to go almost off while signals light up nice and bright. This is an optical illusion... turn signals are definitely brighter than taillight but they are not full brightness. Easy to see taillight dim... almost impossible to realize turn signals are not as bright as they should be.

MarkT GREAT explanation 2 thumbs up. I know about the toilet flushing my son did that on purpose and hot water would scald my a** while I was taking a shower 
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That needs to be a Sticky 

Thanks HT1kid.  Glad my explanation made sense to someone. 

Of course there is not an exact correlation between water flowing through the pipes in your house and electricity flowing through wires to a taillight on a Yamaha, but they are close. 

Voltage in wires is similar to water pressure in pipes.

Amps in wires is similar to water flow rate in pipes.

Resistance in wires, connections, or switches is similar to the flow restriction of water in pipes, pipe connectors, or valves.

One obvious difference is if you try to supply a fire hose nozzle through a pipe the size of a drinking straw instead of from a large water main, it simply won't work, the straw won't flow enough water, no damage done. 

If you try to power a car starter motor through a thin 20 gauge wire instead of a thick 1 gauge cable, starter won't work because the 20 gauge wire can't flow enough amps...  but the small wire will heat up and possibly catch itself or something else on fire!  Major damage could result.